The present invention relates to an electromagnetic flowmeter and, more particularly, to an electromagnetic flowmeter using a ceramic electromagnetic flowmeter measuring pipe.
Electromagnetic flowmeters utilize the Faraday's electromagnetic induction rule and have been popular in a variety of applications which require an output proportional to a flow rate, as well as measurements independent of flow states and fluid constants, and free from pressure loss. In such an electromagnetic flowmeter, a stainless pipe as a nonmagnetic pipe is used as a measuring pipe, and an insulating material such as that sold under the trademark Teflon is coated as a lining on the inside of the pipe. However, the measuring pipe with Teflon lining is expensive and fabrication thereof is complicated. Therefore, ceramic measuring pipes have been in widespread use in recent years.
PCT No. W083/0200 describes such a ceramic measuring pipe. The pipe has coupling flanges at two ends. Radial holes are formed in a direction perpendicular to the coils arranged around the central portion of the outer surface of the pipe. Electrodes are inserted in these holes and the measuring pipe is sintered to fix the electrodes.
In this case, since the measuring pipe is sintered while the electrodes are inserted in the holes, the electrode material must withstand the sintering temperature of ceramic and must have substantially the same thermal expansion coefficient as that of the ceramic. Platinum is an electrode material which satisfies these conditions. However, platinum is expensive and causes an increase in total cost. In order to reduce the cost, it is possible to reduce the electrode diameter. However, an input impedance of an electromagnetic flowmeter converter is increased to adversely affect the operation characteristics of the electromagnetic flowmeter.
In order to solve this problem, a conductive ceramic material can be used as the electrodes. However, such a material normally has a lower electric conductivity than that of a metal, thereby decreasing a detection output level and degrading the operation characteristics of the electromagnetic flowmeter.